Electromagnetic transducer



United States Fatent G My invention relates to electroacoustic-transducers, and in particular to transducers suited to underwater operation.

Electrodynamic` Transducer, now Patent 2,978,671 of V,April 4, 1961.

Underwater transducers for the sonic range usually have eiciencies in the Order of one-tenth to one percent, if they have substantially uniform response over a broad` lfrequency range; they usually have efliciencies of tive to fifty percent, if they are resonant, and this high etliciency` is restricted to a frequencyV range'in the order of onetenth of an octave. Thus, the broad-band transducers are useful normally for receiving sound, where their low eiliciency` is not a great handicap, but areuseless for high-power transmission. The resonant devices, o n the other hand, are not versatile; they fail when bro-ad fre- Underwater sonic (sonar) transducers having The problems of transducer design can be discussed broadly in terms of the following equation:

where dis the velocity of the radiating transducer surface This application is a division of my application-v Ser. No. 241,470, tiled August l1, 1951, land entitled 15 -water transducer in-which stiffness can be negligible.

VIt is, accordingly, an object of thernvention to provide an improved transducer of the character indicated.

It is another object to provide a transducer having high eiiciency over an extended frequency range.

It is a further object to provide an improved nonresonant transducer.

Another object is to provide an improved, relatively highly efficient transducer for electroacoustic or accustoelectric conversion in water and having further useful application in air.

More specifically, it is an object to provide an underwater sonic transducer construction in which R can be largely radiative and can be comparable to wM in magnitude for a relatively wide frequency range.

Also specifically, it s an object to provide an under- It is a general object to provide a transducer construction approaching the ideal of maintaining resistance limiin contact with the4 acoustic medium, P(t) is the timevariable force developed by the transducer due to externally applied power, R is the resistance of the medium plus the mechanical friction and hysteresis of the device, M- is the mass reactance of the medium plus the mass of the moving parts of the transducer, k is the stiffness of the moving parts of the transducer, 'and w is 21r Ytimes the frequency. i i

In the usual underwater-sound transducer, the ruggedness, strengthand'volume of material required for develop'ing large forces leads to a device which is `relatively t massive and stiff. Thus, at all frequencies either the mass or stifness-reactance terms in the denominator are large compared to R. At resonance, these terms are equal in magnitude and hence cancel, leaving R as the term which limits the developed motion." Since R can usually be made `largely radiative, this leads to a highly etlicient device atrresonance ifthe Venergy conversion .mechanism contained in P(t) isfeicient. Hence, under resonant conditions, the device is resistance-limited, and to a large degree, radiation-limited-the ideal condition.

'At frequencies below the resonant frequency, the stiffachieving these results with high power-handling capacity,

large active area, ruggedness, and vadvantageous coupling to the medium. I

Other objects and-various further features of novelty and invention will be pointed out or will occur to those skilled in the `art from a reading of the following specication in conjunction with the accompanying drawings.

In the drawings, which show, for illustrative purposes only, preferred forms of the invention- FIG. 1 is a perspective view, partially broken 'away and sectioned on a plane through the axis of symmetry, schematically showing an under-water transducer incorporating features of the invention;

FIG. 2 is a fragmentary cross sectional view of the transducer of FIG. 1 on an enlarged scale;

FIG. 3 is a view similar to FIG. 1 terna-tive embodiment; and

FIG. 4 is a fragmentary cross sectional view on an'enlarged scale of the embodiment of FIG. 3.

Briey stated, my invention contemplates sonic transducer constructions, particularly underwater transducer but showing an a1- constructions, in which R can be largely radiative and can ance` as the medium in which the transducer is to be emplayed. The active element may be electrically conductive and supported in a magnetic iield, as in a magneticflux gap.

An appreciation of this principle of operation may -be obtained from a theoretical approach, in which one considers the behavior of a sound wave traveling in a medium of 'acoustic resistance plCl and imping'ing normally on the surface of the medium p2C2 (see FIG. .1),' where pzCzis very much smaller than p1C1. The interfacewbetween media is identified by the reference numeralr9. `VIf P1 is the acoustic pressure amplitude of the incident wave, P1 the pressure Aamplitude of the reflected'wave, and P2 Y the transmitted pressure amplitude, and V1, V1', and V2 the corresponding acoustic volume currents (relatedto a of Equation 1)tlien the followingrelationships may be deduced: i

If the medium 1 is water, plCl is 1.5 X105 acoustic ohms. If the medium 2 is air-cell rubber o-r rubber-like material, for which p2 is approximately 0.3, then p2C2 is approximately equivalent to the acoustic resistance of air, i.e., approximately 40 acoustic ohms, and p1C1 is about 3800 times as large as p2C2. Under such conditions, Equation 2 states that P2, the pressure at the boundary, is very small as compared with the pressure P1 in the impinging wave at a distance (a quarter wavelength or more) from the interface or boundary between the two media. Equation 3 shows that the motion U2 in the boundary 9 is twice the motion associated with the incoming wave at a distance. Equation 4 shows that the pressure P1 in the reflected wave at a distance is very much larger than the transmitted pressure, and is oppo` site in phase. be approximately equal to P1 and opposite in sign. Finally, according to Equation 5, U1', the reected current, is approximately one half as large as U2, the current at the boundary, and is opposite in phase.

Equation 4 can be further interpreted to mean that, if the transducer exerts force on the boundary 9 (or creates force in this boundary), then the pressure P2 encountered by this radiating transducer face is transformed by the factor i p la) 1 P202 to the larger radiated pressure P1'. The velocity transformation (Equation may always be characterized by a factor of approximately 2.

In my patent application Ser. No. 241,470, filed August 1l, 1951, and entitled Electrodynamic Transducer, now Patent 2,978,671, I have shown a number of transducer embodiments operating on the principles set forth above. This application is a division of the aforementioned application Ser. No. 241,470 and relates to certain transducer embodiments of the type under discussion which are of circular or arcuate shape.

In the drawings I show embodiments of the invention in connection with transducers which may have an effectively flat, generally circularly shaped, active surface of desired proportions. In these arrangements, the active elements are conductive strips generally designated 10, supported between adjacent parallel magnets generally designated 11. Strips 10 and magnets 11 may be provided in plurality, in an array convenient to the desired overall proportions. The magnets 11 may be of so-called Alnico V material and permanently magnetized so that magnetic-iiux gaps are established between adjacent opposed poles of adjacent magnets at the gaps generally designated 12.

As explained above, I prefer that the active strips 10 be supported at the interface between two media having substantially different acoustic impedances, and in the fonn shown I have provided rings generally designated 13 of sound-attenuating material, such as air-filled rubberlike material, in operative relation to each gap 12, on which the conductive strips 10 are mounted. The conductive strips 10 may be bonded to the sound-attenuatin'g Each active element 10 may be `a single copper strip, or, if necessary, to reduce eddy-current losses, each strip may be a laminated build-up of a plurality of bonded strips. Electrical connections to the strips may be accomplished by placing all strips 10 in parallel, as by connecting one end of all strips to one pole and the other end of all strips to the other pole. However, this would make for unduly low electrical impedance, and I, therefore, prefer the series connection of lall strips 10.

In the specific form shown, optimum usage of the ferromagnetic material, Alnico V, requires that the width of the magnets be substantially twice the gap width, for which condition a magnetic field of 10,000 gausses can be supported in the gaps. Thus, of the total face area of a transducer utilizing Alnico V, substantially one-third may be active. It will be understood that as long as the wavelength is very large compared to the active-element From Equations 2 and 4, P1' is seen to v spacing, this area condition may provide an advantageous lowering of the impedance of the medium (water) presented to the radiating strips 10.

As pointed out previously, the acoustic impedance of airfilled rubber at atmospheric pressure is approximately 3800 times less than the acoustic impedance of water. At 60G-ft. water depth, this ratio is reduced to approximately 200. This change in the impedance ratio need not appreciably affect the mechanics of the device, and hence its performance need not be seriously impaired at moderate depths in water. However, in designing for extremedepth operation, it is important that the magnet thicknesses (and active gap depths) should be adequate to 'allo-w for the compression of the air cell rubber. Thus, for an assumed transducer having magnets l-in. wide by %in. thick, and laminated strips 7/16-in. wide by 0.025-in. thick, the active strips might lie approximately 2 mm. below the outer surfaces of the magnets at atmospheric pressure, and at maximum depths the strips might lie approximately 2 mm. above the back surfaces of the magnets.

If the present transducers are to be used `as high-power projectors, the strips may need to carry currents in the order of amperes per centimeter of width, and hence exert pressures in the orders of dynes/cm.2. Heavy leads and an approximately large transformer may deliver such currents, and power output may be in the order of 1 to 10 kilowatts per square meter oftransducer face throughout the audio range. If, on the other hand, the transducers are to be used as receivers, relatively small transformers can be employed, but the construction of the actual transducer may advantageously be left unchanged.

In this application I illustrate generally circular or arcuate embodiments of transducers operating on the above principles. In lboth illustrated embodiments, generally circular or arcuate magnet elements are radially spaced to define annular magnetic-linx gaps, and the conducting strips 10 are generally circular or arcuate. In FIGS. 1 and 2 a concentric array of magnetized rings 11 is arranged in a series-magnetic circuit, with a suitably formed ferromagnetic means 74 to close the magnetic circuit. A frame member 75 of non-magnetic `material may be embraced by ferromagnetic means 74 and may support all magnetic rings 11. Electrical-conducting means in the form of the rings 10 may be supported in the annular flux gaps between the rings 11 and each of these rings 10 is circumferentially discontinuous, as indicated by the single break 78 in each ring. Heavy solid lines 79-80 schematically indicate an electrical interconnection of the ac tive elements. Air-filled rubber or the like inserts 13 may supp ort the strips 10.

In FIGS. 3 and 4 the circular form of a parallel magnetic circuit is illustrated. The magnetic-flux gaps are again annular, `but they are defined between annular horseshoe magnets and between the poles of each such magnet. Thus, a first annular gap may be defined between the poles 85, 86 of a first annular magnet 8,7; a

second annular gap may be dened between the poles SS-S9 of a second annular magnet 90, and a third annular gap may be defined between adjacent poles 8688 of the adjacent magnets 87--90. A backing plate 91 may hold the magnets together. Arcuate conductor strips may be supported in the gaps on air-lled rubber insei-ts 13, and a series electrical connection of the strips 10 is schematically indicated by leads 95 and jumpers 96.

In the embodiment of FIGS. 1 and 2 a base plate 98 is provided, to which the magnetic structure 10, 74, 7S may be secured in any 'appropriate manner, as by the screws 100, and to which the external sheath is also secured in any appropriate manner. In the embodiment of FIGS. 3 and 4 the external sheath 15 may be secured in any desired manner tothe backing plate 91.

It will be seen Ithat I have described novel acousto-electric and electro-acoustic transducer means applicable to air and water use. The construction provides especially advantageous underwater features, including great powerhandling capacity over a relative broad frequency band. The basic construction is relatively simple and lends itself to arrays of almost any desired coniiguration.

While I have described my invention in detail 'for the preferred forms shown, it will be understood ythat modications may be made within the scope of the invention, as defined in the vfollowing claims.

I claim:

1. An electromagnetic transducer comprising housing means, means including a first medium having substan-k tially the sound-transmitting properties of water and contained by said housing means and dening an acousticresponse face on one side of said housing means, a second medium within said housing means, said second medium being adjacent said irst medium behind said face and having substantially the soundtransmitting properties of air, an array of spaced magnets and magnetic-flux gaps within said transducer andbehind said face, said array comprising generally radially spaced generally arcuate magnets defining a series of generally radially spaced generally arcuate ux gaps therebetween, generally arcuate metallic conducting strips yieldably supported in the magnetic tields of said gaps, said strips being in direct driving relation with said first medium `at the interface between said media, and means making electrical Vconnection with said strips.

2. In a transducer of the character indicated, magnetic means comprising generally radially spaced generally arcuate magnetic elements deiining a series of generally radially spaced generally arcuate linx gaps therebetween, generally arcuate metallic conducting strips located in the fields of said gaps, a medium having substantially the sound-transmitting properties of water on one side of said strips and in intimate contact with said one side, and a medium having substantially the sound-transmitting properties of air on the other side of said strip and in intimate contact with said other side.

3. In a transducer of the character indicated, a Support, magnetic means on said support comprising generally radially spaced generally arcuate magnets dening a series of generally radially spaced generally arcuate flux gaps therebetween, generally arcuate metallic conducting strips in said gaps, a sound transmitting material carried by said support on that side of said strips directed toward the medium in which said transducer is adapted to be immersed and interposed between said strips and said medium, a sound-attentuating material on the other side of said strips, one of said materials supporting said strips in said gap.

4. The transducer of claim 3, in which said soundattentuating material supports said strips in said gap.

5. In a transducer of the character indicated, a support, a plurality of horseshoe magnets of revolution on said support including spaced pole pieces dening between themselves generally radially spaced generally arcuate gaps traversed 'by magnetic linx, generally arcuate metallic conducting strips in said gaps, sound-transmitting material carried by said support on that side of said strips directed toward the medium in which said transducer is adapted to be immersed and interposed between said strips and said medium, and sound-attentuat-.

ing means between said pole pieces on the other side of said strips and supporting said strips in said gaps.

References Cited in the tile of this patent UNITED STATES PATENTS 1,637,397 Sykes Aug. 2, 1927 1,653,128 Smith Dec. 20, 1927 1,732,029 Round Oct. 15, 1929 2,214,591 Massa Sept. 10, 1940 2,249,160 Mott July 15, 1941 2,402,697 Turner June 25, 1946 2,413,012 Turner Dec. 24, 1946 2,419,608 Turner Apr. 29, 1947 2,561,368 Hayes et al. July 24, 1951 2,566,850 VMott Sept. 4, 1951 

